1. Field of the Invention
The present invention relates to an image processing apparatus for reducing random noises of an image by using a recursive filter.
2. Description of the Related Art
A recursive filter represented as follows is known as one example of noise reducers for reducing the random noises of a digital image. EQU Y.sub.i =a.multidot.Y.sub.i-1 +(1-a).multidot.X.sub.i ( 1)
where Y.sub.i is a filtered output of a present frame, Y.sub.i-1 is a filtered output of the last frame, X.sub.i is a live image (an input to the filter) of the present frame, and a is a filter factor.
Thus, the recursive filter reduces the random noise by adding the product of the live image X.sub.i and the filter factor a to the product of the filtered output image Y.sub.i-1 of the last frame and the filter factor (1-a), i.e., by averaging the present image and the last image.
In such a recursive filter, the filter factor a is changed within a range between 0 and 1. If the filter factor a approaches to 0, a weight of the last image approaches to 0 and a weight of the present image approaches to 1. Thus, the influence of the filter is decreased and it can be said that a weak filter is applied to the image. On the contrary, if the filter factor a approaches to 1, the weight of the last image approaches to 1 and the weight of the present image approaches to 0. Thus, the influence of the filter is increased and it can be said that a strong filter is applied to the image. The more the filter factor a appropriates to 1, the more the images are averaged with respect to time and the more the random noises can be reduced.
However, if an image includes an object which is moving and this motion portion of the image is applied with the strong filter having the filter factor a close to 1, the residual image of the last frame appears on the present image or the locus of the moving object appears on the present image. Therefore, the contrast of the image is lowered and the artifact is generated so that the quality of the image is degraded.
For example, in a diagnostic X-ray apparatus for displaying a fluoroscopic image obtained by radiating an X-ray to the patient, if a catheter, a guide wire, and the like are moved in a blood vessel to perform a recovery operation of a constricted portion of the blood vessel and the strong filter having the filter factor a close to 1 is applied to the image, the image of the distal end of the catheter or the guide wire will be obscured by the residual image, resulting in inconvenience in diagnosis.
In order to solve this drawback, there is provided an apparatus in which a movement of the object is detected and the recursive filter processing for every pixel is selectively performed based on the result of detection, i.e., the recursive filter processing is performed only for the still portion and is not for the motion portion. One example of such an apparatus is disclosed in Published Unexamined Japanese Utility Model Application (PUJUMA) No. 63-55400. FIG. 1 shows a block diagram of this prior art.
An image signal from an image pick-up device (not shown) is supplied to an analog-to-digital converter 1. In the case of the diagnostic X-ray apparatus, a TV camera to which a fluoroscopic X-ray image is incident from an image intensifier tube outputs the image signal. The image signal of every frame is converted to a digital image signal X.sub.i. The output of the A/D converter 1 is multiplied with a factor (1-a) by a multiplier 2. The filter factor a is changed within a range between 0 and 1. The output of the multiplier 2 is supplied to a first input terminal of an adder 3. A filtered output Y.sub.i-1 of the last frame, i.e., (i-1)th frame from a frame memory 4 is supplied to a second input terminal of the adder 3 through a multiplier 5. The multiplier 5 multiplies the input signal Y.sub.i-1 with the filter factor a.
An output of the adder 3 (=(1-a) X.sub.i +a.multidot.Yi-1) is supplied to a first terminal I1 of a selector 6. The output of the A/D converter 1 is supplied to a second terminal I2 of the selector 6. The selector 6 selects one of the input signals I1 and I2 based on an output of a comparator 7. The comparator 7 compares the output of the A/D converter 1 with the output of the frame memory 4 for every pixel to obtain the difference between them. The selector 6 outputs the input Il to perform the filter processing when the difference is smaller than a threshold level and the input 12 not to perform the filter processing when the difference is not smaller than the threshold level. The output of the frame memory 4 is output through a digital-to-analog converter 8 to be displayed on a monitor device (not shown).
In this conventional apparatus, the movement of the object is detected based on the difference between the pixel value of the live image X.sub.i output from the A/D converter 1 and the pixel value of the last filtered image Y.sub.i-1 from the frame memory 4. When the movement is detected, the selector 6 is caused to select the input 12 to write the input live image X.sub.i into the frame memory without performing the filter processing. Since the filter processing is performed only for the still portion, it is possible to apply a strong filter in which the filter factor a is set to a large value, e.g., 0.8 or 0.9.
However, the filtered pixels and non-filtered pixels are adjacent to each other at the boundary between the motion portion and the still portion. This boundary markedly appears on the filtered image so that the image becomes unnatural. For example, if there is a motion portion surrounded by a still portion, the still portion is averaged and the pixel values thereof somewhat change but the motion portion is not filtered and the pixel values thereof do not change. Therefore, there is a large difference between the pixel value in the motion portion and that in the still portion, thereby giving an unnatural feeling.
In addition, the threshold value for determining whether or not the filter processing is performed is difficult to properly set. If the threshold level is set too small, the number of pixels subjected to the filter processing is decreased and the influence of filtering is reduced. Therefore, the random noises are hardly reduced. If it is set too large, the number of pixels subjected to the filter processing is increased and the influence of filtering is increased. Therefore, the motion portion is blurred.
As described, in the conventional recursive filter which selectively performs the filter processing for every pixel using the threshold value, it is not possible to satisfy the contradictory requirements of the recursive filter which are to reduce the random noises by averaging the images and to prevent the degradation of the quality of the image due to a lowering of the contrast of the image and a generation of the artifact.